1. Field of the Invention
The invention relates to shape measurement of towed, multi-line acoustic arrays.
2. Description of the Related Art
Towed multi-line acoustic arrays, also known as streamers, are known. Such acoustic arrays are used to detect ships, marine life, marine geology, etc. Streamers also have military applications. The node points of the acoustic array (also referred to as a lattice) are comprised, for example, of hydrophones that receive acoustic energy. The acoustic energy detected by the hydrophones may be generated by the feature detected itself, such as a marine animal. Alternatively, the acoustic energy may be a reflected signal, emitted by an acoustic source, such as a sparker or a boomer.
The data detected by the hydrophones is centrally processed to provide a detection, mapping, etc. of the feature. The hydrophones are connected through an electronic and processing backbone, which serves to coordinate and process the data received by the array. For example, if a boomer is used, timing is coordinated between the chirp of the boomer and the listening of the hydrophones. The waveform and timing of the acoustic energy received by the hydrophones is analyzed by the processing backbone to determine if it represents the same feature. In addition, once a feature is identified, its position can be determined by virtue of the timing and the relative positions of the hydrophones in the array. As is known in the art, this can be readily accomplished via generation of a set of simultaneous equations based upon the distance determined via the time of receipt of the acoustic signal by each hydrophone.
A fundamental problem is that the positions of the hydrophones or other items comprising the nodal points of a towed acoustic array are inherently unstable. Because of currents, position in the array, speed of the boat, or any other of myriad influences, the relative positions of the nodal points change continuously over time. It is thus important to also continually monitor the relative positions of the nodal points of the array. Various systems are used in the art for this purpose. A common system uses a multiplicity of “birds” that clip on the tow lines. Each bird comprises a transducer used for determining the range between nodal points. Thus, the backbone causes one bird to ping and the hydrophones of the nodal points to listen. The measured time of flight (TOF) is used to calculate the distance between the pinging bird and receiving hydrophones. The distances are used along with known quantities (for example, the distance between nodal points on the same line) in a series of simultaneous equations to generate relative distances between nodal points and, thus, the shape of the array.
The accuracy of knowing the shape of the array is highly important to the detection, analysis, etc. by the array. Although there are known systems that perform the above-described determination of the shape of the array, such systems generally rely on match filtering to the shape of the envelope of a continuous wave (CW) tone burst or to a pseudo-random sequence of tone bursts to measure TOF. Matching the shape of the CW leads to a loss of detail, which is then compensated for by using higher frequencies. This, however, results in attenuation, reduction of the range that can be measured, and susceptibility to reflections off of array components, bulkheads, etc. Thus, the present systems can only measure the relative positions of nodal points on the order of 50 cm or greater. Errors of this magnitude result in a degradation in the performance of the array.
In addition, it is noted that the use of CW tone bursts corresponds to use of the resonant frequency of the transducer. This results in a pressure waveform that includes startup and ending transient artifacts that interfere with detection and also uses a relatively narrow detection spectrum centered around the resonant peak.
In addition, as noted above, current systems that use CW tone bursts operating at higher frequencies must locate the emitting transducer external to the streamer hose wall to guarantee a clear acoustic path. This requires special means for attachment, deployment, storage, powering, etc. (for example, using birds as described above).